It is well known that contacting primary alcohols with a suitable catalyst at elevated temperatures (e.g., in excess of 200° C.) causes the alcohol to decompose to hydrogen gas and carbon-containing species. This process is generally known as “alcohol reforming.” For example, methanol reforming leads to the formation of hydrogen and carbon monoxide as illustrated in the following Equation 1:CH3OH→CO+2H2  (1)
The hydrogen produced in the reforming process can then be supplied to a fuel cell in order to produce electric power. The reforming process is endothermic and requires efficient heat transfer to the catalyst, especially in transportation applications (e.g., electric automobiles) where high peak power is necessary, particularly at startup. Methanol reforming is described, for example, by Gunter et al., J. Catal. 203, 133-49 (2001); Breen et al., J. Chem. Soc. Chem. Comm., 2247-48 (1999); European Chemical News, p. 22, (May 11, 1998); and Jiang et al., Appl. Cat. 97A, 145-58 (1993). Methanol reforming and particular application of methanol reforming as a source of hydrogen for fuel cells is described, for example, by Agrell et al., Catalysis-Specialist Periodical Reports, vol. 16, pp. 67-132 (J. J. Spivey, ed., Royal Society of Chemistry, Cambridge, UK, 2002).
It is important to note that carbon monoxide is generally toxic to fuel cell electrodes. For example, fuel cell performance and power economy is typically reduced as the level of carbon monoxide exceeds about 20 ppm in the hydrogen feed. See, Pettersson et al., Int'l J. Hydrogen Energy, vol. 26, p. 246 (2001). It is therefore desirable to convert carbon monoxide to carbon dioxide by reaction with steam as illustrated in the following Equation 2:CO+H2O→CO2+H2  (2)This conversion is known as the water-gas shift reaction and is widely practiced commercially. A description of catalysts, processes and applications of the water-gas shift reaction can be found, for example, in Catalyst Handbook, pp. 283-339 (2nd ed., M. V. Twigg ed., Manson Publishing, London, 1996).
Under conditions similar to those described above with respect to methanol, the reforming of ethanol initially produces acetaldehyde which can then be decomposed (i.e., decarbonylated) to carbon monoxide and methane as illustrated in the following Equation 3:CH3CH2OH→CH3C(O)H+H2→CO+CH4+H2  (3)
As with methanol reforming, ethanol reforming is preferably coupled with the water-gas shift reaction to convert carbon monoxide to carbon dioxide and produce additional hydrogen. Thus, the water-gas shift reaction associated with ethanol reforming produces carbon dioxide, methane and hydrogen as illustrated in the following Equation 4:CO+CH4+H2+H2O→CO2+CH4+2H2  (4)
The most common catalysts for alcohol dehydrogenation and low temperature water-gas shift reactions comprise copper with zinc oxide and sometimes other promoters on a refractory supporting structure, generally alumina or silica. Copper-zinc oxide catalysts, while exhibiting excellent stability for methanol synthesis, have been reported to have inadequate stability for methanol reforming, as described by Cheng, Appl. Cat. A, 130, p. 13-30 (1995) and Amphlett et al., Stud. Surf. Sci. Catal., 139, p. 205-12 (2001).
Most other catalysts reported to be active for alcohol reforming have consisted of metal oxides, usually containing catalytic metals. Yee et al., J. Catal. 186, 279-95 (1999) and Sheng et al., J. Catal. 208, 393-403 (2002) report ethanol reforming over CeO2 by itself or with additional rhodium, platinum or palladium. However, these papers report that ethanol can decompose to a number of unwanted by-products such as acetone, ketene and butene.
Copper-nickel catalysts are known to have a high activity for the dehydrogenation of ethanol. For example, copper-nickel catalysts supported on alumina are active for ethanol reforming. Ethanol reforming over copper-nickel catalysts is described by Mariño et al. in Stud. Surf. Sci. Catal. 130C, 2147-52 (2000) and Freni et al. in React. Kinet. Catal. Lett. 71, 143-52 (2000). Although the references described the catalysts as providing good selectivity for acetaldehyde decarbonylation, each of the references suffered from incomplete conversion and minimal water-gas shift activity at temperatures of 300° C. Further, conventional ethanol reforming catalysts tend to quickly deactivate due to the deposition of carbon on the surface, a process known as coking. At temperatures above 400° C., coking is accelerated by the presence of acid sites on the surface of the catalyst, which promote the dehydration of ethanol to ethylene which then polymerizes. The problem of coking involved with ethanol reforming catalysts is described, for example, by Haga et al. in Nippon Kagaku Kaishi, 33-6 (1997) and Freni et al., in React. Kinet. Catal. Lett., 71, p. 143-52 (2000).
Accordingly, a need persists for improved alcohol dehydrogenation catalysts and processes capable of reforming alcohols at moderate reaction temperatures and with adequate conversion.